Part 2:
A problem was noticed when I changed out the first IC. There was something wrong with the board itself. Take a look at the second
pic. After desoldering, there was no plating on the pads, just discoloured copper pads that won't take solder. The plating or tinning
was no good on the upper side of the board. The entire component side is like this. I changed out I had to use an eraser made for ink to scour the copper enough to take solder. The problem
(not confirmed yet but I'm pretty sure) is likely the way the boards are mated together. There are two pin strips with square pins on the
eprom board that mate with two connectors on the cpu board. The second pic also shows the eprom board connector.
I believe the problem is one or both connectors on the cpu board (now removed) have sprung contacts causing intermittent connection(s)
and loss of control. There was no socket at U22. I desoldered the area to show what the whole board looks like from the component side.
I will again scour the pads with the ink eraser, apply solder to the areas then use my Pace desoldering station to suck up the
solder leaving a tinned area for the 24 pin machine socket. I have a 24 pin dip header that will have its wires soldered to the eprom board.
The strip rows on the eprom board have been removed. As you can see in the third pic, the eprom board doesn't have the same problem as the
cpu board. There will be a secure mechanical connection between the dip header and machine pin socket.
I have seen hundreds of double sided boards but never one with a defect such as this. I wonder how many 8500's with controller problems
have this same issue. I doubt I am the only one with a pc board like this.
The white IC is an 8080A cpu clocked at a whopping 1.700 MHz
Anyway, I am going to wire up the dip header and go from there.